Electrophotographic technology has been widely applied to the field of printers, as well as the field of copiers, due to its immediacy and formation of high-quality images. Electrophotographic photoreceptors (hereinafter, optionally, referred to as “photoreceptor”) lie in the core technology of electrophotography, and organic photoreceptors using organic photoconductive materials have been developed, since they have advantages such as non-pollution and ease in production in comparison with inorganic photoconductive materials.
In general, an organic photoreceptor is composed of an electroconductive support and a photosensitive layer disposed thereon. Photoreceptors are classified into a so-called single-layer photoreceptor having a single photosensitive layer (single photosensitive layer) containing a binder resin dissolving or dispersing a photoconductive material therein; and a so-called multilayered photoreceptor composed of a plurality of laminated layers (laminated photosensitive layer) including a charge-generating layer containing a charge-generating material and a charge-transporting layer containing a charge-transporting material.
In the organic photoreceptor, changes in use environment of the photoreceptor or changes in electric characteristics during repeated use may cause various defects in an image formed with the photoreceptor. In a method as one technique for solving such disadvantages, an undercoat layer containing a binder resin and titanium oxide particles is provided between an electroconductive substrate and a photosensitive layer in order to stably form a good image (for example, refer to Patent Document 1).
The layer of the organic photoreceptor is generally formed by applying and drying a coating liquid prepared by dissolving or dispersing a material in a solvent, because of its high productivity. In such a case, since the titanium oxide particles and the binder resin are incompatible with each other in the undercoat layer, the coating liquid for forming the undercoat layer is provided in the form of a dispersion of titanium oxide particles.
Such a coating liquid has generally been produced by wet-dispersing titanium oxide particles in an organic solvent using a known mechanical pulverizer, such as a ball mill, a sand grind mill, a planetary mill, or a roll mill, by spending a long period of time (for example, refer to Patent Document 1). Furthermore, it is disclosed that when titanium oxide particles are dispersed in a coating liquid for forming an undercoat layer using a dispersion medium, an electrophotographic photoreceptor that exhibits excellent characteristics in repeated charging-exposure cycles even under conditions of low temperature and low humidity can be provided using titania or zirconia as the dispersion medium (for example, refer to Patent Document 2).
The electrophotographic photoreceptor is repeatedly used in an electrophotographic process, i.e., a cycle of charging, exposure, development, transfer, cleaning, neutralization, and the like. In this occasion, since the photoreceptor is repeatedly used, it undergoes various stresses causing deterioration. Examples of such deterioration include chemical damage of the photosensitive layer caused by ozone or NOx, which are highly oxidative, generated from a charging device; and chemical and electrical deterioration caused by a flow of carriers (electric current), which is generated through image exposure, in the photosensitive layer or degradation of the photosensitive layer composition due to neutralization light or external light. In addition, the photoreceptor undergoes mechanical damage, e.g., wear of the photosensitive layer surface, scratching, and delamination, which are caused by friction with a charging roller or a charging brush, which are in contact with the electrophotographic photoreceptor for charging the photoreceptor, a cleaning blade for removing excess toner, a transfer roller for transferring an image, a developer, and paper. In particular, such deterioration occurring on the photoreceptor surface readily affects an image and directly decreases image quality, which is a major cause of limitation of the photoreceptor life.
In a general photoreceptor not having functional layers such as a surface-protecting layer, the photosensitive layer receives these stresses. The photosensitive layer is generally composed of a binder resin and a photoconductive material, and the binder resin substantially determines the strength. However, since the amount of the photoconductive material as a dopant is considerably large, the photoreceptor cannot have sufficient mechanical strength.
Furthermore, a material that can respond to a higher speed of an electrophotographic printing process is required with an increase in demand for high-performance printing. In such a case, the photoreceptor is also demanded to have a good response for shortening the time from exposure to development, in addition to high sensitivity and a long service life.
Furthermore, each layer of the electrophotographic photoreceptor is generally formed by applying a coating liquid containing, for example, a photoconductive material and a binder resin onto a support by dipping, spraying, nozzle coating, bar coating, roll coating, or blade coating. In the process for forming these layers, a coating solution is prepared and applied by a known method in which a material to be contained in a layer is dissolved in a solvent. Furthermore, in many cases, the coating solution is previously prepared and stored.
Examples of the binder resin in the photosensitive layer include vinyl polymers, such as polymethylmethacrylate, polystyrene, polyvinyl chloride, and copolymers thereof; thermoplastic resins, such as polycarbonate, polyester, polysulfone, phenoxy, epoxy, and silicone resins; and various thermosetting resins. Among such a large number of binder resins, the polycarbonate resin shows relatively excellent performance, and various kinds of polycarbonate resins have been developed and practically used (refer to Patent Documents 3 to 6).
On the other hand, it has been reported that an electrophotographic photoreceptor containing a polyarylate resin, which is commercially available under the trade name “U-polymer”, as the binder resin exhibits improved sensitivity compared to that containing a polycarbonate resin (refer to Patent Document 7).
In addition, it has been reported that when a polyarylate resin including a divalent phenol component having a particular structure is used as the binder resin, the coating solution used for producing an electrophotographic photoreceptor exhibits improved mechanical strength and wear resistance, as well as improved stability (refer to Patent Documents 8 and 9).
Furthermore, in the organic photoreceptor, known hole-transporting materials, which are charge-transporting materials, are, for example, hydrazone compounds, triphenylamine compounds, benzidine compounds, stilbene compounds, and butadiene compounds. Known electron-transporting materials, which are charge-transporting materials, are, for example, diphenoquinone compounds.
The charge-transporting material is selected in consideration of characteristics demanded in the photoreceptor. Examples of the characteristics demanded in the photoreceptor include (1) electrostatic charge generated by corona discharge is high in a dark place, (2) attenuation of the charge generated by the corona discharge is low in a dark place, (3) the charge is rapidly dissipated by light irradiation, (4) the residual electric charge after the light irradiation is low, (5) an increase in the residual potential and a decrease in the initial potential are small in repeated use, and (6) changes in the electrophotographic characteristics caused by environmental changes, such as temperature and humidity, are small.
Various charge-transporting materials, such as a hydrazone compound, have been hitherto proposed for improving these characteristics (for example, refer to Patent Documents 10 to 15).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. HEI 11-202519
[Patent Document 2] Japanese Unexamined Patent Application Publication No. HEI 6-273962
[Patent Document 3] Japanese Unexamined Patent Application Publication No. SHO 50-098332
[Patent Document 4] Japanese Unexamined Patent Application Publication No. SHO 59-071057
[Patent Document 5] Japanese Unexamined Patent Application Publication No. SHO 59-184251
[Patent Document 6] Japanese Unexamined Patent Application Publication No. HEI 03-063653
[Patent Document 7] Japanese Unexamined Patent Application Publication No. SHO 56-135844
[Patent Document 8] Japanese Unexamined Patent Application Publication No. HEI 03-006567
[Patent Document 9] Japanese Unexamined Patent Application Publication No. HEI 10-288845
[Patent Document 10] Japanese Patent Publication No. SHO 55-42380
[Patent Document 11] Japanese Patent Publication No. SHO 58-32372
[Patent Document 12] Japanese Unexamined Patent Application Publication No. SHO 61-295558
[Patent Document 13] Japanese Unexamined Patent Application Publication No. SHO 58-198043
[Patent Document 14] Japanese Patent Publication No. HEI 5-42661
[Patent Document 15] Japanese Patent Publication No. HEI 7-21646